1. Field of the Invention
The present invention relates to an image fixing apparatus which can promote saving of energy in fixing and also relates to an image forming apparatus having the image fixing apparatus therein and the image forming process.
2. Description of the Related Art
There are increasingly demands on saving of energy and higher-speed processing for an image forming apparatus according to an electrophotography technique, such as, printers, copiers, facsimiles in recent years. To meet the demands, it is important to improve the thermal efficiency of an image fixing apparatus configured for fixing a toner image carried onto a recording medium by means of an image transferring process or a direct image-carrying process.
As an image fixing apparatuses for fixing a toner image, image fixing apparatuses according to a contact heating process, such as, a heat roller process, a film-heating process, and an electro magnetic induction heating process have been widely used.
An image fixing apparatus according to a heat roller process is basically configured to have a pair of rotatable rollers which comprises a fixing roller having interiorly a heat source, such as, a halogen lamp, to control the temperature to a given one, and a pressure roller brought into pressure contact with the fixing roller. In the fixing apparatus, a recording medium having a not-fixed toner image is sent into the contact portion between the pair of rotatable rollers, so-called fixing nip portion, to melt the not-fixed toner image by means of heat and pressure actions from the fixing roller and the pressure roller to fix the image.
An image fixing apparatus according to a film-heating process has been proposed in, for instance, Japanese Patent Application Laid-Open (JP-A) Nos. 63-313182, and 01-263679. These image fixing apparatuses are configured to make a recording medium having a not-fixed toner image thereon attached firmly to a heater which is mounted and supported by a supporting member through a thin fixing film having thermal resistance to feed heat from the heater to the recording medium through the fixing film while sliding and moving the fixing film to the heater. As for the heater, for example, a ceramic heater which comprises a resistive layer disposed on a ceramic substrate having properties, such as, thermal resistance, insulation, and high thermal conductivity, like an alumina and an aluminum nitride, is used. Because a thin fixing film having a low thermal capacity can be used for the fixing film, an image fixing apparatus according to a film-heating process has a higher efficiency of heat transfer and achieves a shorter warm-up time than those of an image fixing apparatus according to a heat roller process, and it enables a quick start and saving of energy.
As to an image fixing apparatus according to an electromagnetic induction heat process, for instance, there is one described in Japanese Patent Application Laid-Open (JP-A) No. 08-22206, in which a technique that the Joule heat is induced by eddy currents generated on a magnetic metal member by an alternating magnetic field to make a heater including the magnetic metal member heating through the use of electromagnetic induction is used.
Hereinafter, the image fixing apparatus disclosed in the JP-A 08-22206 will be illustrated referring to FIG. 1. An image fixing apparatus shown in FIG. 1 comprises a film inner surface guide 103 with a heater 102 mounted thereon, the heater 102 comprising an exciting coil unit 100 and an magnetic metal member 101 which is a heating section; a cylindrical film 104 having thermal resistance and wrapping around the film inner surface guide 103 in a state where the magnetic metal member 101 is held in contact with the inside wall thereof; and a pressure roller 105 which is arranged to face the magnetic metal member 101 and brought into pressure contact with the external wall of the film 104 to form a fixing nip portion N between the film 104 and itself and also rotates the film 104.
As for the film 104, a single layer having a film thickness of 100 μm or less, preferably 50 μm to 20 μm or more and thermal resistance, made from a PTFE (polytetrafluoroethylene), a PFA (perfluoroalkoxy), a FEP (fluorinated ethylene propylene) and the like, or a multilayer in which a film made from a PTFE, a PFA, and a FEP and the like coated on the outer circumference of a film made from a polyimide, a polyamideimide, a PEEK (polyether ether ketone), a PES (polyethersulfone), a PPS (polyphenylene sulfide) and the like, are used.
The film inner surface guide 103 comprises a member having rigidity and thermal resistance, formed with a PEEK resin, a PPS resin, and the like, and the heater 102 is engaged into generally the central part of the bottom surface of the film inner surface guide 103 longitudinally along the guide.
The pressure roller 105 comprises a core 105a and a heat-resistant rubber layer 105b which is coated around the core 105a and has excellent demolding properties, such as, a silicone rubber. The pressure roller 105 is arranged with a given pressing force so that it is brought into contact with the magnetic metal member 101 of the heater 102 through the film 104 by means of a bearing unit and a biasing unit (both not shown). The pressure roller 105 is driven to rotate in a counterclockwise direction indicated with the arrow by a drive unit (not shown).
A frictional force is generated between the pressure roller 105 and the film 104 by the rotating drive unit of the pressure roller 105 to lead the film 104 to rotate along with the pressure roller 105, and the film 104 rotates in a sliding manner with a state where it is attached firmly to the magnetic metal member 101 of the heater 102.
In the condition that the heater 102 reaches a given temperature, a recording medium P which is carrying a not-fixed toner image T formed in the image forming section (not shown) is sent to the fixing nip portion N located between the film 104 and the pressure roller 105. The recording medium P is transported to the fixing nip portion N in a state being sandwiched between the pressure roller 105 and the film, and in the course of the transport, heat from the magnetic metal member 101 is given to the recording medium P through the film 104 to lead a the not-fixed toner image T to be melted and fixed on the recording medium P. At the outlet of the fixing nip Portion N, the recording medium P passed through is separated from the surface of the film 104 to be transported to the output tray (not shown).
As described above, an image fixing apparatus according to an electromagnetic induction heating process allows the magnetic metal member 101 as an heat induction unit to be arranged close by the toner image T on the recording medium P through the film 104 by means of generation of eddy currents, and this enables a heat efficiency higher than that of an image fixing apparatus according to a film-heating process.
However, in the above noted image fixing apparatuses according to a heat roller process, a film-heating process, and an electromagnetic induction heating process, it is very difficult to take a sufficient nipping time because rollers are used in order to press a recording medium on a fixing member. To ensure a fixing within the short nipping time, the heat temperature should be set to a needlessly high temperature, which has become a subject to resolve.
Particularly in a system in which the above-noted heater 102 is used, it is impossible to take a large nip width because pressure activities are carried out using the pressure roller 105 made from a rubber, although the heater 102 having a contact surface in a shape of plane plate to be contact with the recording medium P is used. Namely, the configuration stated above makes it impossible to enlarge the nip width unless the diameter of the pressure roller is enlarged, since the rubber member is compressed and transformed by the crimping with the pressure roller 105 to press the recording medium P and the film 104 on the surface of the heater 102 to form the nip portion. Besides, there is a disadvantage that a repulsive force of the rubber portion differs at the central part of and at the end part of the contact surface with the pressure roller 105 which is made from a rubber, and a crimping force at the end part of the contact face weaker than that of the central part. With an increase in a roller diameter of the pressure roller 105, the mass of the pressure roller 105 becomes greater, and this results in a deprivation of heat needed for fixing at the time of nipping, which requires an excessively increased temperature of the heater 102. There is no way of resolving the differences in a pressure force between at the central part and at the end part of the pressure roller 105 made from a rubber, in principle. Therefore, time for undergoing a sufficient pressure force becomes shorten. To make the toner particles deformed and make the toner layer fused with and attached firmly on the surface of the recording medium P, it is required to extract fixing performance by excessively increasing the toner temperature to make the toner viscosity decreased.
It is not preferable that an excessive application of heat like this causes an increased consumption of energy in terms of energy saving since an amount of heat more than needed is needlessly given to the recording medium P and the temperature of the heater 102 becomes higher due to the increased heat temperature, which causes an increased amount of heat radiation, an increased temperature of the film 104 itself, and an increased amount of heat diffusion to the surroundings.
Further, because of raising the toner temperature in a small amount of time, the toner temperature at the portion being in contact with the film 104 extremely differs in temperature from that of the portion being in contact with the recording medium P, and the toner temperature of the portion being in contact with the recording medium P is low and becomes to have a viscosity barely enough to be fixed on the recording medium P, but the toner temperature of the portion being in contact with the film 104 has a problem that it is liable to cause an offset phenomenon because it has exceedingly high temperature and shows a sharp decrease in the viscosity of the toner.